1. Field of the Invention
The invention relates to a telemetry device and can be applied especially to a system for the detection of obstacles, especially obstacles constituted by wire-shaped objects such as high-voltage cables. It relates notably to obstacle detection systems for helicopters.
The presence of cables or wire-shaped objects makes the piloting of helicopters at low altitudes a very vulnerable operation. This is especially true at night when the standard means of vision, of the infrared camera or light intensifier type, cannot be used to detect such targets.
2. Description of the Prior Art
Many approaches have been proposed to resolve the problem of the detection of such obstacles. Some of these approaches may be cited by way of non-restrictive examples.
The article by P. Condom, Evitement d'obstacles pour helicopteres: une solution en ondes millimetriques (Obstacle Avoidance For Helicopters: A Solution Using Millimetric Waves) in Interavia 5, 1985, pp. 459-460, describes a millimetric wave radar.
This system has the drawback of showing reduced sensitivity when the angle of incidence between the cable and the line of sight is great. This limitation must be considered in direct relation to the range of the wavelength used. For millimetric wavelengths, it is the specular reflection that is preponderant. This means that there are no lateral scattering cones.
Measurements of laser equivalent surfaces of electrical cables at the wavelengths of the infrared range have shown that this range of wavelengths is well suited to the detection of cables. Indeed, the scattered reflection makes it possible to obtain echos at the high values of incidence.
A CO.sub.2 laser emitting at the 10.6 .mu.m wavelength can be used as an active source. The LOTAWS project may be cited here. The detection of cables is done at this wavelength. This project is described in a U.S. Army report entitled: "Laser Obstacle Terrain Avoidance Warning System", R&D technical report ECOM-0145.3, ECOM-72-0145.4, ECOM-0145.2, U.S. Army Electronics Command, Fort Monmouth, N.J., 1972-1973. However, problems related to the amount of space occupied considerably restrict the use of a laser source such as this on board a helicopter.
Devices based on a television type scanning system have also been proposed. A first device of this type is described in the article by M. Eibert: "Laser Radar Based Obstacle Avoidance System For Helicopters", ERF91-20, pp. 185-193. This device uses laser diodes emitting at 0.8 .mu.m.
The scanning of the line of sight is obtained mechanically and electronically with television type scanning pattern. The electronic scanning is obtained in one direction by the use of an array of diode lasers that emit sequentially. Similarly, at reception, an array of detectors is used, working in phase with the array of laser diodes. The scanning in the other direction is obtained mechanically by means of an oscillating mirror. This device has the drawback of using multiplexing techniques to place the emitting diode and the corresponding detector temporally in phase. Furthermore, the mechanical scanning, namely the oscillating mirror, introduces accumulations of dots at the ends.
A second device of this type is obtained in a sales brochure of the firm Northrop. This system is called OASYS (Obstacle Avoidance System) and is described in the article by B. Nordwall, "Army Seeks To Detect Wires, So Helicopter Pilots Can Fly Fast, Low"; Aviation Week & Space Technology, Jan. 22, 1990, pp. 77 to 81.
The device uses a laser diode emitting on a wavelength of 0.9 .mu.m. The scanning of the line of sight is obtained with a holographic scanning. The pattern described is also a television type scanning pattern. This system implies the use of a hologram having a diameter that is twice that of the reception pupil. Only the equivalent of 25% of the power, therefore, is recovered at the reception pupil.
Passive detection systems may also be cited. In this case, the cable is detected by means of the magnetic field created by the current. The detection is therefore possible only when the electrical cables are under voltage, and this considerably limits the field of use. A method such as this is described in the article by K. E. Potter et al., "Technique For Overhead Wire Detection", IEEE Proc., Vol. 128, No. 7, December, 1981, pp. 427-432.